Fuel cells have recently received attention as an important technique which can achieve high energy efficiency and significantly reduce emission of CO2. The type of fuel cell varies with the electrolyte used. Fuel cells for industrial application fall into four types: a phosphoric-acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), and a polymer electrolyte fuel cell (PEFC). Among them, the SOFC is characterized in that it exhibits small internal resistance and therefore the highest power generation efficiency in the fuel cells, as well as that because there is no need to use any precious metal as a catalyst, its production costs can be kept down. For these reasons, the SOFC is a system widely applicable from small-scale applications, such as those for domestic use, to large-scale applications, such as a power plant, and expectations have been raised for its potential.
The FIG. shows the structure of a general planar SOFC. A general planar SOFC includes a cell in which an electrolyte 1 made of ceramic material, such as yttria-stabilized zirconia (YSZ), an anode 2 made such as of Ni/YSZ, and a cathode 3 made such as of (La,Ca)CrO3 are layered and integrated. The planar SOFC further includes: a first support substrate 4 adjoining the anode 2 and having passages of fuel gas (fuel channels 4a) formed therein; and a second support substrate 5 adjoining the cathode 3 and having passage of air (air channels 5a) formed therein, wherein the first and second support substrates 4, 5 are fixed to the top and bottom, respectively, of the cell. The first support substrate 4 and the second support substrate 5 are fixed to the cell so that their gas passages are perpendicular to each other. The first support substrate 4 and the second support substrate 5 are made of metal, such as SUS.
In the planar SOFC having the above structure, a fuel gas, such as hydrogen (H2), town gas, natural gas, biogas or liquid fuel, is allowed to flow through the fuel channels 4a and concurrently air or oxygen (O2) is allowed to flow through the air channels 5a. During this time, the cathode develops a reaction of 1/2O2+2e−→O2−, while the anode develops a reaction of H2+O2−→H2O+2e−. These reactions cause direct conversion of chemical energy to electric energy, so that the planar SOFC can generate electric power. To provide high power, in an actual planar SOFC, a plurality of cell structure units shown in the FIG. are layered.
In producing the planar SOFC having the above structure, hermetic sealing is necessary between its members (particularly, each support substrate and the cell) to prevent the occurrence of gas leakage. For this purpose, there has been proposed a method for hermetically sealing the members by interlaying a sheet-shaped gasket made of inorganic material, such as mica, vermiculite or alumina, between the members. However, this method does not involve bonding the members together, which may cause a tiny amount of gas leakage and thus result in poor fuel use efficiency. Therefore, consideration has been given to a method for bonding the members with an adhesive material made of glass.
To bond high-expansion members made of metal or ceramics, it is necessary to conform the coefficient of thermal expansion of the adhesive material to those of the members. In addition, to achieve good bondability, the adhesive material is required to have sufficient fluidity at the bonding temperature. Furthermore, the temperature range of the SOFC in which it develops an electrochemical reaction (i.e., the operating temperature range) is as high as about 600 to about 800° C. and the SOFC is operated at these temperatures over a long period. Therefore, the adhesive material is required to have high thermal resistance to avoid, even when exposed to high temperatures for a long period, deterioration in hermeticity and bondability due to melting of bonded portions and degradation in power generation property of the fuel cell due to evaporation of glass components.
To achieve the above required properties, glass compositions capable of precipitating high-expansion crystals through thermal treatment are proposed in Patent Literatures 1 and 2. Specifically, Patent Literature 1 describes a crystallizable glass composition which can precipitate CaO—MgO—SiO2-based crystals through thermal treatment. Patent Literature 2 describes a crystallizable glass composition which can precipitate MgO-based crystals when undergoing thermal treatment. Furthermore, Patent Literature 3 describes an adhesive material made of a SiO2—B2O3—SrO-based amorphous glass composition precipitating no crystals through thermal treatment.